Head massive, wide and depressed. Snout partially naked with or without fleshy cylindrical tentacles branched or not, its contour semicircular. In males naked area is wider but does not reach nares or orbits. Females have only a narrow naked band on snout edge. Nostrils juxtaposed and closer to eyes than to tip of snout. Eyes in dorsal lateral position, orbits not raised and without odontodes. Interorbital space broad and flat. Supraoccipital flat without ridges, posterior border straight and truncate. Movable hypertrophied cheek odontodes well developed, specimens can have 10–12. Size of these odontodes variable in each specimen, the longest odontode nearly extends to pectoral-fin origin independently of size of specimen. Anteriorly, bases of movable hypertrophied odontodes have covering of plates. Opercular bone has exposed surface easy visible externally, its lateral margin carries odontodes.

Mouth oval or rounded. Upper lip narrow, usually covering premaxilla and only external surface is visible, edge is almost horizontal and with very minute undulations. Internal surface papillose. Lower lip broad, its border with very minute undulations. Lower lip surface papillose. Papillae smaller near border of lip increasing in size near lower jaws. Papillae of anterior lip have similar size to those near lower jaws. Maxillary barbels not present in the holotype, in male paratypes short and free, and in females paratypes very short leaving only tip free. Upper and lower jaws of similar length. Hemimandibles straight, sometimes placed horizontally or forming an open V between them. Teeth numerous and minute. Between 30–45 teeth in each hemimandible. Premaxillary and dentary teeth of same size. Teeth incisor type, asymmetrically bifid, medial cusp longer and wider than lateral cusp. Medial cusp rounded or straight truncated, lateral cusp pointed. Tooth apex curved toward interior of mouth. Tooth apex yellowish, stalk whitish. Premaxillary and dentary without posterior papillae or ornamentation.

Lateral line plates 24–25. Post-anal plates 11–12. Inter-dorsal plates six or seven, just in front of the spine of adipose-fin there are two plates with keels. The dorsal-fin origin is anterior to vertical passing through pelvic-fin origin. Dorsal-fin with one spine, followed by seven branched rays; when depressed their tips do not reach adiposefin origin. Adipose-fin well developed and always present. Spine of adipose fin is wide, and a little bit curved toward the distal end. Pectoral-fin with one spine, and six branched rays. When depressed, pectoral-fin spine reaches posterior to first third, (females) or half, (males) of pelvic-fin spine length. Spine of pectoral-fin is a bit shorter than first branched ray. Distal region of pectoral-fin spine with enlarged odontodes and small fleshy prominence. Pelvic-fin with one spine and five branched rays; its posterior margin surpassed anal-fin base when depressed. Anal-fin with one flexible spine and four branched rays. Caudal-fin rays i,14,i. Posterior border of caudal-fin obliquely truncate.

Sexual dimorphism. On snout, adult males have wide naked dorsolateral area. Naked area reaches anterior region of eyes and nares. Over this area, soft and fleshy tentacles are found, small, with few or no branches ( Fig. 1View FIGURE 1). Tentacles are arranged in the following pattern: On the snout margin tentacles are disposed in a line along edge. Those at center are larger and may have branched tips. From tip of the snout, a dorsomedial row of four tentacles runs backward reaching to just anterior to nares. These tentacles are conical, and some may have branched tips. Just anterior to nares, line of tentacles bifurcates with three on each side or become wider with six tentacles oriented transversely ( Fig. 1View FIGURE 1). Females with very narrow naked band at anterior edge of snout and a row of small and conical fleshy tentacles; the two central tentacles slightly longer ( Figs. 2View FIGURE 2, 3View FIGURE 3).

Color. Head and body brown to black or dark grey with evident or diffuse white to whitish dots. Belly is lighter with white to whitish, faded dots. All fins brown or black with dots or irregular bands, whitish, spaced over rays and/or interradial membrane. When dots are on rays only, the interradial membrane is translucent. The border of dorsal and caudal-fin is black. Dorsal-fin with a black spot between bases of the spine and the first branched ray. Tips of dorsal and ventral principal caudal-fin rays whitish or orange ( Figs. 1View FIGURE 1, 2View FIGURE 2). In live specimen head and body yellowish brown or yellowish green with well-defined whitish or orange dots. Dorsal, pectoral and pelvic fins have same color of body; spines have transverse brown or dark bands, and the branched rays with whitish or orange dots or bands, interradial membranes translucent or with irregular whitish dots. Caudal-fin has transverse dark or brown bands and/or whitish dots on branched rays ( Fig. 4View FIGURE 4).

Geographical distribution. Ancistrus shuar is represented at the MEPN by four specimens in three cataloged lots. The specimens were caught in the Santiago River of Morona-Santiago province ( Fig. 8View FIGURE 8).

Etymology. Dedicated to ancient and brave Shuar, indigenous group that live in the Morona-Santiago province. It is considered a noun in apposition.